two battery or not two battery

Having two batteries only benefits your system if the car is not running.  If you're alternator cant handle that much wattage then the battery will drain and then after awhile the second battery would drain as well. Get a new alternator if your car cant handle that much wattage. A quick way to check to see if itll work is take the amp rating on your alternator and multiply by 40% or .40.  That'll tell you how much reserve power you have with the stock alt (power that is left over after running AC and lighs and engine and what not).  Then take the RMS of all your amps and double it to account for inefficiency (just to be safe).  then divide that by 13.8 (average voltage of alternators).  Now depending on how loud you play your music dvide it by 3 because the average music signal is a third less than a test tone, and that should give you all your amps current draws.  Now compare the two numbers.  If the reserve power is less than the current draw you're screwed get a new alternator.   

If anyone disagrees with this please let me know cause im still learning more everyday from this site.

dkiley,

If your lights are not dimming it sounds like you system is getting the proper juice from the alternator/battery and you don't need to upgrade.

The majority of the time when you need to upgrade is when your lights dim while playing your music.  The dimming of the lights exploits your systems need for more current.  But, since you are not seeing the symptoms of a weak electrical system it seems that you don't need to do anything unless you want to.

You should be upgrading the wires from the alternator to at least a 4 guage line as customsuberb recommended. You still need to feed current to the system and the smaller type wires that are stock usually restrict this.

Now for the second battery. Lots of people use them, most use them incorrectly. If you are going to be using two batteries and have them wired together with no isolator, then the batteries need to be of the exact same type / model and preferably serial number incrementation. This is due to the fact that no two batteries are alike. One battery will always be the stonger one, this battery will cause nothing but headaches for the weaker the one and will eventually cause it to fail.

If you are going to keep a two battery system, the yellow top in the back and the factory up front, invest in a battery isolator to totally separate the two batteries. The alternator will charge the same, the isolator will provide the charge to the battery that needs it. Like was pointed out above, one of the main reasons for a multiple battery system is for extended playtime when the vehicle is not running. Some higher output systems will require much more current than a single battery / alternator can supply, these are the systems that use batteries that are all of the same type.

As far a cap goes, save your money. There are very few instances where a cap is actually beneficial.